A gas turbine generally includes a compressor section, a combustion section having a combustor and a turbine section. The compressor section progressively increases the pressure of the working fluid to supply a compressed working fluid to the combustion section. The compressed working fluid is routed through and/or around an axially extending fuel nozzle that extends within the combustor. A fuel is injected into the flow of the compressed working fluid to form a combustible mixture. The combustible mixture is burned within a combustion chamber to generate combustion gases having a high temperature, pressure and velocity. The combustion gases flow through one or more liners or ducts that define a hot gas path into the turbine section. The combustion gases expand as they flow through the turbine section to produce work. For example, expansion of the combustion gases in the turbine section may rotate a shaft connected to a generator to produce electricity. The turbine may also drive the compressor by means of a common shaft or rotor.
The temperature of the combustion gases directly influences the thermodynamic efficiency, design margins, and resulting emissions of the combustor. For example, higher combustion gas temperatures generally improve the thermodynamic efficiency of the combustor. However, higher combustion gas temperatures may increase the disassociation rate of diatomic nitrogen, thereby increasing the production of undesirable emissions such as oxides of nitrogen (NOx) for a particular residence time in the combustor. Conversely, a lower combustion gas temperature associated with reduced fuel flow and/or part load operation (turndown) generally reduces the chemical reaction rates of the combustion gases, thereby increasing the production of carbon monoxide (CO) and unburned hydrocarbons (UHCs) for the same residence time in the combustor.
In order to balance overall emissions performance while optimizing thermal efficiency of the combustor, certain combustor designs include multiple fuel injectors that are arranged around the liner downstream from the primary combustion zone. The fuel injectors deliver a second fuel/air mixture radially through the liner to provide for fluid communication into the combustion gas flow field. This type of system is commonly known in the art and/or the gas turbine industry as an axial fuel staging (AFS) system.
In operation, a portion of the compressed working fluid is routed through and/or around each of the fuel injectors and into the combustion gas flow field. A liquid or gaseous fuel from the fuel injectors is injected into the flow of the compressed working fluid to provide a second combustible mixture, which spontaneously combusts in a secondary combustion zone as it mixes with the hot combustion gases. The introduction of the combustible mixture into the secondary combustion zone increases the firing temperature of the combustor and, because the fuel injectors are downstream of the primary combustion zone, the combustion gases from the primary combustion zone have a first residence time, and the combustion gases from the secondary combustion zone have a second (shorter) residence time. As a result, the overall thermodynamic efficiency of the combustor may be increased without sacrificing overall emissions performance.
One challenge with injecting a liquid fuel into the combustion gas flow field using existing AFS systems is that the momentum of the combustion gases generally inhibits adequate radial penetration of the liquid fuel into the combustion gas flow field. For this reason, local evaporation of the liquid fuel may occur along an inner surface of the liner at or near the fuel injection point, thereby resulting in a high temperature zone and high thermal stresses. Another challenge associated with liquid fuel injectors is a tendency for the fuel injectors to coke at even moderately elevated temperatures.
Therefore, an improved system for injecting a liquid fuel into the combustion gas flow field for enhanced mixing would be useful.